1. Field of the Invention
The present invention relates to subterranean well cementing operations, and more particularly, to dispersant and fluid loss control additives for well cements, well cement compositions containing the additives and methods of using the compositions.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casings and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein which supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions are also utilized in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks or holes in pipe strings and the like.
Cement composition dispersants which also reduce fluid loss are often used in well cement compositions with or without additional fluid loss control agents. Such dispersants are extensively used to reduce the apparent viscosities of the cement compositions in which they are utilized. The reduction of the viscosity of a cement composition allows the cement composition to be pumped with less friction pressure and utilizing less pump horsepower. In addition, the lower viscosity often allows the cement composition to be pumped in turbulent flow. Turbulent flow characteristics are desirable when pumping cement compositions in wells to more efficiently remove drilling fluid from surfaces in the well bore as the drilling fluid is displaced by the cement composition being pumped. The inclusion of dispersants in cement compositions is also desirable in that the presence of the dispersants facilitates the mixing of the cement compositions and reduces the water required for the cement compositions. Cement compositions having reduced water content are characterized by improved compressive strength development.
Fluid loss control agents are used in well cement compositions to reduce fluid loss from the cement compositions to permeable formations or zones into or through which the cement compositions are pumped. In primary cementing, the loss of fluid, i.e., water, to permeable subterranean formations or zones can result in premature gelation of the cement composition whereby bridging of the annular space between the permeable formation or zone and the pipe string being cemented prevents the cement composition from being placed over the entire length of the annulus.
A number of dispersing agents have been utilized heretofore in well cement compositions. For example, certain organic acids such as gluconic acid and citric acid have been recognized by those skilled in the art as well cement dispersants. However, such organic acids also function as strong cement composition set retarding agents which is often undesirable. That is, the presence of an organic acid dispersant in a cement composition prevents the cement composition from setting for a relatively long period of time which is often costly or otherwise detrimental.
Another cement composition dispersant which also exhibits fluid loss control properties in well cement compositions is the condensation product of formaldehyde, acetone and an alkali metal sulfite which is commercially available under the trade designation "CFR-3.TM." from Halliburton Energy Services, Inc. of Duncan, Okla. While this and other dispersants function very well in well cement compositions, they are often environmentally unacceptable in offshore well operations in that they do not undergo complete biodegradation in the environment and can cause damage to aquatic and other life therein.
Thus, there are continuing needs for improved biodegradable cement composition dispersant and fluid loss control additives, improved well cement compositions containing such additives and methods of using the cement compositions.